Using N-Terminal Coordination of Cu(II) and Ni(II) to Isolate the Coordination Environment of Cu(I) and Cu(II) Bound to His13 and His14 in Amyloid-β(4-16)

Investigating Cu(I) binding to model peptides of N-terminal Aβ isoforms

Amyloid beta (Aβ) peptides and copper (Cu) ions are each involved in critical biological processes including antimicrobial activity, regulation of synaptic function, angiogenesis, and others. Aβ binds to Cu and may play a role in Cu trafficking. Aβ peptides exist in isoforms that vary at their C-and N-termini; variation at the N-terminal sequence affects Cu binding affinity, structure, and redox activity by providing different sets of coordinating groups to the metal ion. Several N-terminal isoforms have been detected in human brain tissues including Aβ1-40/42, Aβ3-42, pEAβ3-42, Aβ4-42, Aβ11-40 and pEAβ11-40 (where pE denotes an N-terminal pyroglutamic acid). Several previous works have individually investigated the affinity and structure of Cu(I) bound to some of these isoforms' metal binding domains. However, the disparately reported values are apparent constants collected under different sets of conditions, and thus an integrated comparison cannot be made. The work presented here provides the Cu(I) coordination structure and binding affinities of these six biologically relevant Aβ isoforms determined in parallel using model peptides of the Aβ metal binding domains (Aβ1-16, Aβ3-16, pEAβ3-16, Aβ4-16, Aβ11-16 and pEAβ11-16). The binding affinities of Cu(I)-Aβ complexes were measured using solution competition with ferrozine (Fz) and bicinchoninic acid (BCA), two colorimetric Cu(I) indicators in common use. The Cu(I) coordination structures were characterized by X-ray absorption spectroscopy. The data presented here facilitate comparison of the isoforms' Cu-binding interactions and contribute to our understanding of the role of Aβ peptides as copper chelators in healthy and diseased brains.

Exploration of the Potential Role of Serum Albumin in the Delivery of Cu(I) to Ctr1

Human serum albumin (HSA) is the major copper (Cu) carrier in blood. The majority of previous studies that have investigated Cu interactions with HSA have focused primarily on the Cu(II) oxidation state. Yet, cellular Cu uptake by the human copper transport protein (Ctr1), a plasma membrane-embedded protein responsible for Cu uptake into cells, requires Cu(I). Recent in vitro work has determined that reducing agents, such as the ascorbate present in blood, are sufficient to reduce the Cu(II)HSA complex to form Cu(I)HSA and that Cu(I) is bound to HSA with pM affinity. The biological accessibility of Cu(I)HSA suggests that HSA-bound Cu(I) may be an unappreciated form of Cu cargo and a key player in extracellular Cu trafficking. To better understand Cu trafficking by HSA, we sought to investigate the exchange of Cu(I) from HSA to a model peptide of the Cu-binding ectodomain of Ctr1. In this study, we used X-ray absorption near-edge spectroscopy to show that Cu(I) becomes more highly coordinated as increasing amounts of the Ctr_1-14 model peptide are added to a solution of Cu(I)HSA. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to further characterize the interaction of Cu(I)HSA with Ctr_1-14 by determining the ligands coordinating Cu(I) and their bond lengths. The EXAFS data support that some Cu(I) likely undergoes complete transfer from HSA to Ctr_1-14. This finding of HSA interacting with and releasing Cu(I) to an ectodomain model peptide of Ctr1 suggests a mechanism by which HSA delivers Cu(I) to cells under physiological conditions.

The metal ion hypothesis of Alzheimer's disease and the anti-neuroinflammatory effect of metal chelators

Alzheimer's disease (AD), characterized by the β-amyloid protein (Aβ) deposition and tau hyperphosphorylation, is the most common dementia with uncertain etiology. The clinical trials of Aβ monoclonal antibody drugs have almost failed, giving rise to great attention on the other etiologic hypothesis regarding AD such as metal ions dysmetabolism and chronic neuroinflammation. Mounting evidence revealed that the metal ions (iron, copper, and zinc) were dysregulated in the susceptible brain regions of AD patients, which was highly associated with Aβ deposition, tau hyperphosphorylation, neuronal loss, as well as neuroinflammation. Further studies uncovered that iron, copper and zinc could not only enhance the production of Aβ but also directly bind to Aβ and tau to promote their aggregations. In addition, the accumulation of iron and copper could respectively promote ferroptosis and cuproptosis. Therefore, the metal ion chelators were recognized as promising agents for treating AD. This review comprehensively summarized the effects of metal ions on the Aβ dynamics and tau phosphorylation in the progression of AD. Furthermore, taking chronic neuroinflammation contributes to the progression of AD, we also provided a summary of the mechanisms concerning metal ions on neuroinflammation and highlighted the metal ion chelators may be potential agents to alleviate neuroinflammation under the condition of AD. Nevertheless, more investigations regarding metal ions on neuroinflammation should be taken into practice, and the effects of metal ion chelators on neuroinflammation should gain more attention. Running title: Metal chelators against neuroinflammation.

Sequence-Activity Relationship of ATCUN Peptides in the Context of Alzheimer's Disease

Amino-terminal Cu^II and Ni^II (ATCUN) binding sequences are widespread in the biological world. Here, we report on the study of eight ATCUN peptides aimed at targeting copper ions and stopping the associated formation of reactive oxygen species (ROS). This study was actually more focused on Cu(Aβ)-induced ROS production in which the Aβ peptide is the "villain" linked to Alzheimer's disease. The full characterization of Cu^II binding to the ATCUN peptides, the Cu^II extraction from Cu^II(Aβ), and the ability of the peptides to prevent and/or stop ROS formation are described in the relevant biological conditions. We highlighted in this research that all the ATCUN motifs studied formed the same thermodynamic complex but that the addition of a second histidine in position 1 or 2 allowed for an improvement in the Cu^II uptake kinetics. This kinetic rate was directly related to the ability of the peptide to stop the Cu^II(Aβ)-induced production of ROS, with the most efficient motifs being HWHG and HGHW.

Why the Ala-His-His Peptide Is an Appropriate Scaffold to Remove and Redox Silence Copper Ions from the Alzheimer’s-Related Aβ Peptide

The progressive, neurodegenerative Alzheimer’s disease (AD) is the most widespread dementia. Due to the ageing of the population and the current lack of molecules able to prevent or stop the disease, AD will be even more impactful for society in the future. AD is a multifactorial disease, and, among other factors, metal ions have been regarded as potential therapeutic targets. This is the case for the redox-competent Cu ions involved in the production of reactive oxygen species (ROS) when bound to the Alzheimer-related Aβ peptide, a process that contributes to the overall oxidative stress and inflammation observed in AD. Here, we made use of peptide ligands to stop the Cu(Aβ)-induced ROS production and we showed why the AHH sequence is fully appropriate, while the two parents, AH and AAH, are not. The AHH peptide keeps its beneficial ability against Cu(Aβ)-induced ROS, even in the presence of Zn^II-competing ions and other biologically relevant ions. The detailed kinetic mechanism by which AHH could exert its action against Cu(Aβ)-induced ROS is also proposed.

Multimodal imaging of hemorrhagic transformation biomarkers in an ischemic stroke model

Hemorrhagic transformation of ischemic stroke has devastating consequences, with high mortality and poor functional outcomes. Animal models of ischemic stroke also demonstrate the potential for hemorrhagic transformation, which complicates biochemical characterization, treatment studies, and hinders poststroke functional outcomes in affected subjects. The incidence of hemorrhagic transformation of ischemic stroke in animal model research is not commonly reported. The postmortem brain of such cases presents a complex milieu of biomarkers due to the presence of healthy cells, regions of varying degrees of ischemia, dead and dying cells, dysregulated metabolites, and blood components (especially reactive Fe species released from lysed erythrocytes). To improve the characterization of hemorrhage biomarkers on an ischemic stroke background, we have employed a combination of histology, X-ray fluorescence imaging (XFI), and Fourier transform infrared (FTIR) spectroscopic imaging to assess 122 photothrombotic (ischemic) stroke brains. Rapid freezing preserves brain biomarkers in situ and minimizes metabolic artifacts due to postmortem ischemia. Analysis revealed that 25% of the photothrombotic models had clear signs of hemorrhagic transformation. The XFI and FTIR metabolites provided a quantitative method to differentiate key metabolic regions in these models. Across all hemorrhage cases, it was possible to consistently differentiate otherwise healthy tissue from other metabolically distinct regions, including the ischemic infarct, the ischemic penumbra, blood vessels, sites of hemorrhage, and a region surrounding the hemorrhage core that contained elevated lipid oxidation. Chemical speciation of deposited Fe demonstrates the presence of heme-Fe and accumulation of ferritin.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Synthesis and structural characterization of copper–cuprizone complexes

The chemistry of copper with cuprizone has challenged chemists for over 70 years. We characterize the classical ‘blue’ product, containing Cu(iii) and hydrolyzed cuprizone, and a green multimeric Cu(ii) product, containing unhydrolyzed cuprizone.

Oxidase Reactivity of CuII Bound to N-Truncated Aβ Peptides Promoted by Dopamine

The redox chemistry of copper(II) is strongly modulated by the coordination to amyloid-β peptides and by the stability of the resulting complexes. Amino-terminal copper and nickel binding motifs (ATCUN) identified in truncated Aβ sequences starting with Phe4 show very high affinity for copper(II) ions. Herein, we study the oxidase activity of [Cu–Aβ_4−x] and [Cu–Aβ_1−x] complexes toward dopamine and other catechols. The results show that the Cu^II–ATCUN site is not redox-inert; the reduction of the metal is induced by coordination of catechol to the metal and occurs through an inner sphere reaction. The generation of a ternary [Cu^II–Aβ–catechol] species determines the efficiency of the oxidation, although the reaction rate is ruled by reoxidation of the Cu^I complex. In addition to the N-terminal coordination site, the two vicinal histidines, His13 and His14, provide a second Cu-binding motif. Catechol oxidation studies together with structural insight from the mixed dinuclear complexes Ni/Cu–Aβ_4−x reveal that the His-tandem is able to bind Cu^II ions independently of the ATCUN site, but the N-terminal metal complexation reduces the conformational mobility of the peptide chain, preventing the binding and oxidative reactivity toward catechol of Cu^II bound to the secondary site.

Molecular Insight into Cu2+-Induced Conformational Transitions of Amyloid β-Protein from Fast Kinetic Analysis and Molecular Dynamics Simulations

Cu²⁺-mediated amyloid β-protein (Aβ) aggregation is implicated in the pathogenesis of Alzheimer's disease, so it is of significance to understand Cu²⁺-mediated conformational transitions of Aβ. Herein, four Aβ mutants were created by using the environment-sensitive cyanophenylalanine to respectively substitute F4, Y10, F19, and F20 residues of Aβ₄₀. By using stopped-flow fluorescence spectroscopy and molecular dynamics (MD) simulations, the early stage conformational transitions of the mutants mediated by Cu²⁺ binding were investigated. The fast kinetics unveils that Cu²⁺ has more significant influence on the conformational changes of N-terminal (F4 and Y10) than on the central hydrophobic core (CHC, F19, and F20) under different pH conditions (pH 6.6-8.0), especially Y10. Interestingly, lag periods of the conformational transitions are observed for the F19 and F20 mutants at pH 8.0, indicating the slow response of the two mutation sites on the conformational transitions. More importantly, significantly longer lag periods for F20 than for F19 indicate the conduction of the transition from F19 to F20. The conduction time (difference in lag period) decreases from 4.5 s at Cu²⁺ = 0 to undetectable (<1 ms) at Cu²⁺ = 10 μM. The significant difference in the response time of F19 and F20 and the fast local conformational changes of Y10 imply that the conformational transitions of Aβ start around Y10. MD simulations support the observation of hydrophobicity increase at N-terminal during the conformational transitions of Aβ-Cu²⁺. It also reveals that Y10 is immediately approached by Cu²⁺, supporting the speculation that the starting point of conformational transitions of Aβ is near Y10. The work has provided molecular insight into the early stage conformational transitions of Aβ₄₀ mediated by Cu²⁺.

Impact of N-Truncated Aβ Peptides on Cu- and Cu(Aβ)-Generated ROS: CuI Matters!

In vitro Cu(Aβ_1-x )-induced ROS production has been extensively studied. Conversely, the ability of N-truncated isoforms of Aβ to alter the Cu-induced ROS production has been overlooked, even though they are main constituents of amyloid plaques found in the human brain. N-Truncated peptides at the positions 4 and 11 (Aβ_4-x and Aβ_11-x ) contain an amino-terminal copper and nickel (ATCUN) binding motif (H₂ N-Xxx-Zzz-His) that confer them different coordination sites and higher affinities for Cu^II compared to the Aβ_1-x peptide. It has further been proposed that the role of Aβ_4-x peptide is to quench Cu^II toxicity in the brain. However, the role of Cu^I coordination has not been investigated to date. In contrast to Cu^II , Cu^I coordination is expected to be the same for N-truncated and N-intact peptides. Herein, we report in-depth characterizations and ROS production studies of Cu (Cu^I and Cu^II ) complexes of the Aβ_4-16 and Aβ_11-16 N-truncated peptides. Our findings show that the N-truncated peptides do produce ROS when Cu^I is present in the medium, albeit to a lesser extent than the unmodified counterpart. In addition, when used as competitor ligands (i.e., in the presence of Aβ_1-16 ), the N-truncated peptides are not able to fully preclude Cu(Aβ_1-16 )-induced ROS production.

Exploration of the Potential Role for Aβ in Delivery of Extracellular Copper to Ctr1

Amyloid beta (Aβ) peptides are notorious for their involvement in Alzheimer's disease (AD), by virtue of their propensity to aggregate to form oligomers, fibrils, and eventually plaques in the brain. Nevertheless, they appear to be essential for correct neurophysiology on the synaptic level and may have additional functions including antimicrobial activity, sealing the blood-brain barrier, promotion of recovery from brain injury, and even tumor suppression. Aβ peptides are also avid copper chelators, and coincidentally copper is significantly dysregulated in the AD brain. Copper (Cu) is released in significant amounts during calcium signaling at the synaptic membrane. Aβ peptides may have a role in maintaining synaptic Cu homeostasis, including as a scavenger for redox-active Cu and as a chaperone for clearing Cu from the synaptic cleft. Here, we employed the Aβ_1-16 and Aβ_4-16 peptides as well-established non-aggregating models of major Aβ species in healthy and AD brains, and the Ctr_1-14 peptide as a model for the extracellular domain of the human cellular copper transporter protein (Ctr1). With these model peptides and a number of spectroscopic techniques, we investigated whether the Cu complexes of Aβ peptides could provide Ctr1 with either Cu(II) or Cu(I). We found that Aβ_1-16 fully and rapidly delivered Cu(II) to Ctr_1-14 along the affinity gradient. Such delivery was only partial for the Aβ_4-16/Ctr_1-14 pair, in agreement with the higher complex stability for the former peptide. Moreover, the reaction was very slow and took ca. 40 h to reach equilibrium under the given experimental conditions. In either case of Cu(II) exchange, no intermediate (ternary) species were present in detectable amounts. In contrast, both Aβ species released Cu(I) to Ctr_1-14 rapidly and in a quantitative fashion, but ternary intermediate species were detected in the analysis of XAS data. The results presented here are the first direct evidence of a Cu(I) and Cu(II) transfer between the human Ctr1 and Aβ model peptides. These results are discussed in terms of the fundamental difference between the peptides' Cu(II) complexes (pleiotropic ensemble of open structures of Aβ_1-16 vs the rigid closed-ring system of amino-terminal Cu/Ni binding Aβ_4-16) and the similarity of their Cu(I) complexes (both anchored at the tandem His13/His14, bis-His motif). These results indicate that Cu(I) may be more feasible than Cu(II) as the cargo for copper clearance from the synaptic cleft by Aβ peptides and its delivery to Ctr1. The arguments in favor of Cu(I) include the fact that cellular Cu export and uptake proteins (ATPase7A/B and Ctr1, respectively) specifically transport Cu(I), the abundance of extracellular ascorbate reducing agent in the brain, and evidence of a potential associative (hand-off) mechanism of Cu(I) transfer that may mirror the mechanisms of intracellular Cu chaperone proteins.

Designed Metal-ATCUN Derivatives: Redox- and Non-redox-Based Applications Relevant for Chemistry, Biology, and Medicine

The designed “ATCUN” motif (amino-terminal copper and nickel binding site) is a replica of naturally occurring ATCUN site found in many proteins/peptides, and an attractive platform for multiple applications, which include nucleases, proteases, spectroscopic probes, imaging, and small molecule activation. ATCUN motifs are engineered at periphery by conjugation to recombinant proteins, peptides, fluorophores, or recognition domains through chemically or genetically, fulfilling the needs of various biological relevance and a wide range of practical usages. This chemistry has witnessed significant growth over the last few decades and several interesting ATCUN derivatives have been described. The redox role of the ATCUN moieties is also an important aspect to be considered. The redox potential of designed M-ATCUN derivatives is modulated by judicious choice of amino acid (including stereochemistry, charge, and position) that ultimately leads to the catalytic efficiency. In this context, a wide range of M-ATCUN derivatives have been designed purposefully for various redox- and non-redox-based applications, including spectroscopic probes, target-based catalytic metallodrugs, inhibition of amyloid-β toxicity, and telomere shortening, enzyme inactivation, biomolecules stitching or modification, next-generation antibiotic, and small molecule activation.

How trimerization of CTR1 N-terminal model peptides tunes Cu-binding and redox-chemistry

Employing peptide-based models of copper transporter 1 (CTR1), we show that the trimeric arrangement of its N-terminus tunes its reactivity with Cu, promoting Cu(ii) reduction and stabilizing Cu(i). Hence, the employed multimeric models of CTR1 provide an important contribution to studies on early steps of Cu uptake by cells.

Electrocatalytic Reduction of CO2 to Ethylene by Molecular Cu-Complex Immobilized on Graphitized Mesoporous Carbon

The electrochemical reduction of carbon dioxide (CO₂ ) to hydrocarbons is a challenging task because of the issues in controlling the efficiency and selectivity of the products. Among the various transition metals, copper has attracted attention as it yields more reduced and C2 products even while using mononuclear copper center as catalysts. In addition, it is found that reversible formation of copper nanoparticle acts as the real catalytically active site for the conversion of CO₂ to reduced products. Here, it is demonstrated that the dinuclear molecular copper complex immobilized over graphitized mesoporous carbon can act as catalysts for the conversion of CO₂ to hydrocarbons (methane and ethylene) up to 60%. Interestingly, high selectivity toward C2 product (40% faradaic efficiency) is achieved by a molecular complex based hybrid material from CO₂ in 0.1 m KCl. In addition, the role of local pH, porous structure, and carbon support in limiting the mass transport to achieve the highly reduced products is demonstrated. Although the spectroscopic analysis of the catalysts exhibits molecular nature of the complex after 2 h bulk electrolysis, morphological study reveals that the newly generated copper cluster is the real active site during the catalytic reactions.